n = input()
m=0
count=0
if 999<int(n)<10000:
    for left in range(3):
        for right in range(left+1, 4):
                num = int(n[left:right+1])
                for i in range(2, num//2):
                    if num%i==0:
                        count=+1
                if count==0:
                    if m<num and num!=1:
                        m = num
if m!=0:
    print(m)
else:
    print("0")
                    
